The invention relates to improvements in apparatus for and in methods of making a rod of fibrous material. More particularly, the invention relates to improvements in a method of and in an apparatus for making a continuous rod of fibrous material from a stream which is obtained by showering fibrous material into a first portion of an elongated path and the stream is conveyed past a trimming device which removes the surplus of fibrous material to thus convert the stream into a rod-like filler which is ready to be draped in a web of cigarette paper or other suitable wrapping material to form therewith a rod which is ready to be subdivided into plain cigarettes, cigars, cigarillos or filter rod sections of desired length.
It is well known to monitor the density of a tobacco stream and to utilize the thus obtained signals for adjustment of the trimming device when the signals indicate that the density of the stream is excessive or too low. As a rule, the density monitoring means comprises a source of corpuscular radiation which is caused to penetrate into successive increments of the trimmed or equalized stream, especially into successive increments of the draped filler, and an ionization chamber or another suitable transducer which ascertains the quantity of radiation that has penetrated across the stream and is indicative of the density of the corresponding increments of the stream. Such monitoring means are quite reliable because their measurements are not influenced by certain parameters of the fibrous material, such as the color of tobacco shreds. However, since the nuclear monitoring means is remote from the trimming device (as mentioned above, such monitoring means is normally placed adjacent the wrapped filler), the density of the conveyed fibrous material is likely to be influenced by the wrapping mechanism which is located between the trimming device and the monitoring means and/or by other components of a rod making machine which contact and/or otherwise influence the advancing mass of fibrous material intermediate the trimming device and the transducer of the nuclear monitoring means. Moreover, many manufacturers of cigarettes or other rod-shaped articles of the tobacco processing industry are reluctant to employ monitoring means which rely on corpuscular radiation because the presence of such monitoring means necessitates numerous expensive undertakings in order to ensure the safety of attendants.
Certain more recent proposals involve the utilization of density monitoring devices which employ infrared light. Reference may be had to German Offenlegungsschrift No. 36 24 236. An important advantage of density monitoring devices which employ infrared light is that their space requirements are minimal so that such devices can be placed into close or immediate proximity of the trimming device, i.e., their density measurements are not distorted by the wrapping mechanism and/or by any other component parts of the rod making machine. Moreover, such density monitoring devices are less expensive because they do not require any safety measures such as those which are not only necessary but actually prescribed by authorities if the density of the stream is measured by nuclear monitoring means. Therefore, when the density is measured by an infrared monitoring device, the number of rejects (rod-shaped articles which have been separated from the rod, which have undergone a final test for the presence of defects including unsatisfactory density of their fillers, and which have been found to be defective because the density of their fillers is excessive or too low) is minimal. This will be readily appreciated since the density is monitored immediately adjacent the locus of removal of the surplus from the moving stream, and the results of such measurements can be used to adjust the trimming device if the monitored density deviates from the desired optimum density.
A drawback of infrared density monitoring devices is that the accuracy of their measurements is dependent upon certain parameters, such as the color of fibrous material which forms the stream. Thus, the intensity or other characteristics of signals which are transmitted by the transducers of infrared monitoring devices are likely to change if the color of the conveyed material has changed, even though the density of the conveyed material does not change at all. Attempts to overcome such drawbacks of infrared density monitoring devices include simultaneous utilization of such devices with nuclear density monitoring means. Signals from the ionization chamber of the nuclear density monitoring means are used to test the signals which are furnished by the transducer of an infrared monitoring device to thus ensure that adequate measures can be undertaken when the characteristics of signals which are transmitted by the transducer of the infrared monitoring device deviate from the normally more reliable signals from the ionization chamber of the nuclear density monitoring means. Such apparatus are quite satisfactory; however, their cost is high because of the need for simultaneous monitoring of the density by a nuclear monitoring means and by an infrared density monitoring device. In addition, it is necessary to carry out all safety measures which are prescribed when one employs monitoring means operating with corpuscular radiation in spite of the fact that the safety provisions are or can be somewhat less stringent because the nuclear density monitoring means can employ a relatively weak source of corpuscular radiation.